This invention relates to wavelength dependent optical couplers useful in bidirectional signal transmission with optical fibers and more particularly to a method of providing a dichroic element for such a coupler.
Generally, an optical beam splitter or coupler operates to divide an incident beam of light into two beams for application to two circuits. Such devices are employed in wavelength duplexing in fiber optic communications systems to increase information capacity, provide security among different communications channels and in general, in accomplishing bidirectional transmission over a single fiber.
Dichroic filters have been employed with such fibers to distinguish between or separate two optical wavelengths. According to present technology, dichroic transmission and reflection properties are achieved using multilayer dielectric coatings. Hence, by the use of suitable dielectric coatings or layers, one can achieve dichroic reflectors with both transmission and reflection properties being highly dependent upon both wavelength and polarization. Such dichroic filters operate to selectively reflect and transmit light as a function of wavelength and relatively independent of the plane of vibration.
Typical materials which may be deposited or evaporated on substrates to provide dichroic filters include zinc sulfide, titanium dioxide, magnesium flouride and other materials as well.
The problems of coatings including dichroic layers with optical devices as beam splitters and couplers has been investigated by the prior art. In my copending patent application entitled BIDIRECTIONAL COUPLER FOR COMMUNICATION OVER A SINGLE FIBER, Ser. No. 136,636, filed on Apr. 2, 1980 and assigned to the Assignee herein, many problems concerning dichroic coatings have been explained and characterized. In view of such problems, the above described copending application attempts to circumvent such problems by providing a bulk surface device which is desirably extremely thin, as between fifty to seventy-five micrometers. To provide a dichroic filter of such dimensions, a glass substrate fabricated from an optical quality glass as a fused silica SiO2 is provided having an initial thickness of about one hundred twenty-five to one hundred fifty micrometers.
A multilayer dielectric dichroic coating is applied to one of the large surface areas of the glass substrate. The dichroic coating is a hot coating to provide increased adhesion and durability. After deposition of the coating, the dichroic filter is ground and polished to a nominal thickness. In such an operation, the dichroic surface is protected by applying a soluble adhesive to the dichroic coating, while grinding and polishing the glass. The composite structure is then diced into either square or rectangular wafers by means of a diamond saw. The size of the wafer is variable, but is larger than the fiber to which it is to be ultimately applied. A nominal cross section of two millimeters by two millimeters can be employed. The composite dichroic filter is positioned over a polished fiber beam splitter half. A thin layer of less than twenty-five micrometers of optical epoxy is applied to the fiber beam splitter half and the dichroic wafer is directly attached by means of the epoxy to the half. In such configurations, it is important that the epoxy layer be quite thin such that axial displacement is small to achieve a minimum coupling loss.
As indicated, the above noted patent application describes a beam splitter device including methods for applying a dichroic wafer to an optical fiber. In any event, it is a prime desire in formulating such optical devices to reduce the axial displacement between coupler halves. By reducing axial displacement, one is enabled to achieve an increase in the transmission coupling efficiency.
As indicated, the prior art technique requires the use of a high quality glass substrate upon which the dichroic coating is deposited. The glass substrate has a relatively high index of refraction and is relatively expensive. It is, of course, understood that the above described dichroic filter is a composite device as including a dichroic layer deposited upon a glass substrate.
It is an object of the present invention to entirely eliminate the glass substrate and to provide a dichroic coating which is extremely thin as having a thickness of less than thirty micrometers, which dichroic coating is directly applied to the face of a fiber by means of an extremely thin layer of epoxy. Therefore and in accordance with such structure, one achieves a dichroic filter which is extremely thin as containing no glass substrate and which, therefore substantially reduces the axial displacement between coupler halves to obtain an improved transmission coupling efficiency. These advantages are provided together with the further advantages that the methods employed eliminate the need for a high quality glass substrate of relatively stringent optical requirements and hence, as will be described, the methods according to this invention employ inexpensive substrates as part of the formulation process.